In recent years, numerous technologies concerning water have been developed, and among them, the use of membrane separation methods is widely spread in various fields owing to such features as energy saving, space saving, labor saving and higher product quality.
The separation membranes used for water treatment can be classified into two major groups; nano filtration membranes (NF membranes)/reverse osmosis membranes (RO membranes) and microfiltration membranes (MF membranes)/ultrafiltration membranes (UF membranes). The former is used for removing salt, ions, etc. from seawater and brine water, and on the other hand, the latter is used in water purification processes for producing industrial water and tap water from river water, groundwater and treated-sewage water. Further, sewage and industrial wastewater which have been treated by activated sludge methods are now actively treated by methods called “membrane separation activated sludge methods (=membrane bioreacters (MBR))” having MF membranes/UF membranes directly immersed in activated sludge tanks.
In the recent situation where water shortage is acute and chronic, water treatment methods using these membranes are further technically developed, and in recent years, numerous fresh water production facilities employing the technology called the integrated membrane system (IMS) in which fresh water is efficiently produced by removing organic matter and fine particles in seawater or brine water using MF membranes/UF membranes or purifying sewage or industrial wastewater by MBR as pretreatment and subsequently treating with RO membranes are constructed in the Middle East, Asia and other regions suffering from water shortage.
Present systems for producing fresh water from seawater and brine water include, for example, technologies in which the pretreatment by sand filtration as a conventional water purification technology is followed by the treatment using NF membranes/RO membranes, and also methods in which the pretreatment of seawater or brine water using MF membranes/UF membranes is followed by the treatment using NF membranes/RO membranes as described before. In these systems, since the salt of seawater cannot be removed by the pretreatment, the removal of salt entirely relies on the latter treatment using NF membranes/RO membranes. Accordingly in the membrane treatment method of using NF membranes/RO membranes requiring a supply pressure higher than the osmotic pressure, a pump called “booster pump” must be used for pressurization in order to supply raw water to NF membranes/RO membranes. That is, if the salt concentration of the raw water supplied to NF membranes/RO membranes is higher, the osmotic pressure is higher, and consequently high pressurization by a booster pump is required to consume much energy for operating the booster pump.
In order to solve these problems, membrane treatment systems in which advanced sewage treatment and seawater desalination are integrated, described in non-patent documents 1 and 2, were developed.
According to these technologies, after sewage is treated by MBR, RO membranes are used to produce fresh water. Further, the concentrated water produced when the RO membrane treatment is performed is mixed with seawater. Therefore, fresh water can be produced more efficiently than before, and the salt concentration in the seawater treated by the RO membranes can be lowered for allowing the pressurization by the booster pump to be reduced more than before when the booster pump is used to operate the RO membrane treatment used for seawater desalination. Thus, a more energy-saving system can be realized.
In these technologies, it is supposed that the RO membrane-concentrated water produced as a byproduct from the sewage treatment line for treating sewage with MBR and RO membranes is joined as it is with the seawater supplied to a seawater desalination treatment line. Therefore, each of the non-patent documents 1 and 2 shows a treatment flowchart as shown in FIG. 1. FIG. 1 is a flowchart of the conventional integrated sewage treatment-seawater desalination system described in each of the non-patent documents 1 and 2. In FIG. 1, water (A) undergoing treatment (sewage) is treated by a pretreatment apparatus (1) (MBR) to decompose the organic matter and to separate and remove suspended components, fine particles, etc., for obtaining treated water. Further, the treated water is subjected to RO membrane treatment in a first semipermeable membrane treatment apparatus (2) (RO membrane treatment apparatus) on the treatment line side for the water (A) undergoing treatment, to obtain produced water (fresh water) and concentrated water. In the technologies described in the abovementioned documents, the concentrated water obtained here is made to join the treatment line for water (B) undergoing treatment, and mixed with the water (B) undergoing treatment (seawater), for decreasing the osmotic pressure of the water (B) undergoing treatment. The water (B) undergoing treatment mixed with the concentrated water is subjected to RO membrane treatment in a second semipermeable membrane treatment apparatus (3) (RO membrane treatment apparatus), to obtain produced water (fresh water) and concentrated water. Subsequently the produced water (fresh water) obtained in the first semipermeable membrane treatment (2) is joined with that obtained in the second semipermeable membrane treatment apparatus (3), to be used for any of various applications as fresh water.
However, in these technologies, in the case where the pretreated water obtained by pretreating sewage, industrial wastewater or seawater by MF membranes/UF membranes is used as raw water, the amount of the treated water may change hour by hour or day by day. In the treatment flow of FIG. 1, the piping (4) of the concentrated water flowing from the first semipermeable membrane treatment apparatus (2) is merely connected with the piping (5) of the water (B) undergoing treatment, and the flow rate of the concentrated water and the flow rate of the water (B) undergoing treatment are not controlled. Further, after the concentrated water and the water (B) undergoing treatment are joined, there is no means provided for mixing the two types of water.
In the conventional treatment flow as described above, in the case where the aforementioned amount of the treated water changes, when the RO membrane-concentrated water produced as a byproduct from the sewage treatment line and seawater are mixed, the mixing ratio changes to change the water quality such as salt concentration (osmotic pressure) supplied to the RO membranes on the seawater treatment line side after mixing. If the water quality such as salt concentration of feed water changes, the load acting on the RO membranes becomes large, not allowing stable treatment, and the life of the membranes is likely to be shortened. Further, depending on the specifications of the second semipermeable membrane treatment apparatus, there is also a problem that the operation of the second semipermeable membrane treatment apparatus (3) must be entirely or partially suspended. Furthermore, in the flowchart of each of the non-patent documents 1 and 2, since no positively mixing means is installed at a position downstream of the joint of the two pipings, the mixed water is not homogeneously mixed, and concentration-polarized (osmotic pressure-polarized) water is likely to be supplied to the RO membranes. Also in this case, the load acting on the RO membranes becomes large, not allowing stable treatment, and the life of membranes is likely to be shortened. If the life of membranes becomes short, there is such a problem that more frequent maintenance and RO membrane exchange become necessary. Further, as described above, though depending on the specifications of the second semipermeable membrane treatment apparatus (3), there may also be another problem that the operation of the second semipermeable membrane treatment apparatus (3) must be entirely or partially stopped.
Meanwhile, in a water treatment system in which raw water is pretreated and supplied to RO membranes, it is for example known that after sand filtration treatment is performed as pretreatment, the treated water is stored in a pretreated water tank, and that a supply pump and a high pressure pump are used to feed the raw water to RO membranes or that an inverter is used to control the feed rate of the high pressure pump (patent document 1).
Further, in the case where RO membranes are connected in series to perform seawater desalination, it is known that when the concentrated water produced by treating seawater by a first stage of RO membranes is supplied to a second stage of RO membranes, a valve for adjusting the flow rate thereof or the operation pressure or a relay water tank is installed (patent document 2).
Further, in the case where RO membranes are connected in series as in patent document 2, to perform seawater desalination, it is known that an RO membrane module consisting of two RO membrane elements is provided as a first stage of RO membranes, and that after the water treated by one of the RO membrane elements is mixed with that of the other RO membrane element, the mixed water is supplied to a second stage of RO membranes, when a flow regulating valve is used for control (patent document 3).
These means can be used to adjust the flow rate of the raw water supplied to the latter stage of RO membrane treatment.
However, in a fresh water production system in which different types of raw water are mixed as described before, the amount of the raw water supplied from the previous stage of treatment can change to change or heterogenize the water quality after mixing. Therefore, it is difficult to solve the aforementioned problems merely by regulating the flow rate using a regulating valve or installing a water tank.